HF alkylation is an established process in the petroleum refining industry which is used on a wide scale for the production of high quality, high octane gasoline from lower boiling feeds. Commercial refinery plants usually operate with an isoparaffin stream, predominantly isobutane, which is alkylated with C.sub.3 to C.sub.4 olefins to form branched chain paraffin products boiling in the gasoline range, typically up to about 385.degree. F. (about 197.degree. C.). A major source of the olefin chargestock is the light olefin fraction from fluid catalytic cracking (FCC units), comprising principally C.sub.3 -C.sub.4 olefins; the olefins in this fraction, especially the isobutene, readily alkylate iso-butane in the presence of the HF alkylation catalyst to form isoparaffins which constitute a high octane gasoline boiling range product. The UOP and Phillips HF alkylation processes, which together provide substantially the entire alkylation capacity of the United States refining industry are described in Handbook of Petroleum Refining Processes, R. A. Meyers (Ed.), Chemical Process Technology Handbook Series, McGraw-Hill, New York 1986, ISBN 0-07-041763-6, to which reference is made for a description of these processes.
Two types of HF alkylation unit are in general use at the present time. In one type, the gravity flow type reactor, the hydrocarbon reactants meet the liquid hydrofluoric acid entering the bottom of the reactor from an acid cooler to which the acid flows from an acid settler after the alkylation reaction has taken place. The driving force for the circulation of the acid and the hydrocarbon reactants is the difference in density between the catalyst and the hydrocarbons at different points in the system aided by the jet action of the injection nozzles in the reactor. The acid settler permits a phase separation to take place between the denser acid phase and the lighter hydrocarbon phase. The acid phase is recycled to the acid cooler and then back to the reactor; the hydrocarbon phase including the alkylation product is fed to a fractionation section where the propane and unreacted isobutane are separated from the motor fuel alkylate fraction. The isobutane is recycled and propane is removed from the unit. A unit of this type is described in U.S. Pat. No. 3,716,343, to which reference is made for a description of the unit and its mode of operation.
The other principal type of unit currently in use is the pumped acid flow type in which the mixed hydrocarbon feed is introduced into the reactor through spargers along the vertical length of the reactor. From the reactor the catalyst and the hydrocarbons flow into an acid settler where a phase separation takes place in the same way as in the gravity flow unit, permitting product and catalyst recovery in the same manner as described above. Compared to the gravity flow reactor, the pumped circulation reactor uses a smaller inventory of acid because of the higher circulation speed of the catalyst in this type of unit and the smaller size of the piping utilised in the unit.
The petroleum refining industry has always recognised the potential for hazard created by HF alkylation units and has consistently ensured, by the superior mechanical and metallurgical specifications and operating practices utilised in the design, construction and operation of these units, that a high level of safety has been achieved; as a result, the HF alkylation process has enjoyed an almost unparalleled record of industrial safety. The industry has, however, been concerned that the intrinsic safety of these units should be enhanced to secure a higher level of potential operating safety and to guard against the consequences of an uncontrolled release of unit contents, however unlikely this may be. The potential magnitude of the risks inherent in operating an HF alkylation unit may be reduced by decreasing the inventory of acid in the unit and in one of its embodiments, the present invention enables the acid inventory to be reduced by a significant amount, even compared to the lower inventories used in the pumped circulation type of unit. In addition, the present invention enables the intrinsic safety factor of both the gravity flow and pumped circulation units to be increased by providing for the acid inventory to be stored at a remote, safe location away from the main body of the unit. The acid is stored in this location without risk of release during routine operation and during maintenance activity on the unit and can be evacuated to this safe location within a very short time if any uncontrolled release from the unit should occur.